The invention relates generally to gas turbines and more specifically to control of gas turbine operation at baseload under cold fuel conditions.
Industrial-based turbines are often gas-fired and are typically used at power plants to drive generators and produce electrical energy. Combustion dynamics in the combustors of such turbines are defined as pressure pulsations within the combustion system caused by feedback between non-steady heat release and combustion system characteristics. Two such characteristics are chamber acoustics and the fuel delivery system. Combustion dynamics at very high levels can be very destructive and may result in the forced outage of the power plant.
The problem of combustion dynamics is known and is typically controlled to acceptable levels through a number of techniques, including geometric optimization, variation of fuel introduction location and quantity, and fuel properties. With an existing combustor system geometry, splitting the fuel delivery percentages among the various fuel valves introducing fuel into the combustor the problem of combustion dynamics can be abated.
It will be appreciated that there are a number of different types of fuel gases for the combustors of turbines, including natural gas, LPG's such as propane and butane, refinery gases and coal-derived gases. The energy content of each of these fuels varies with its source and, of course, there are variations in energy content among the various types of fuels. The temperature of the fuel gas supplied to the combustor can also be quite different from system to system. For example, many power plants generating electricity from the output of gas turbines provide a fuel gas heater to provide a constant fuel gas temperature to the combustor. Other sites may have a number of boost compressors to elevate the temperature. Thus, different sites provide fuel gas at different temperatures and pressure. Furthermore, sites may source fuel gas from several different vendors or distribution points, which implies that both the temperature and composition of the fuel gas can vary.
The standards for setting fuel gas composition and are defined by a parameter called the Wobbe Index. The modified Wobbe Index allows comparison of the energy content of different fuel gases at different temperatures. Since the gas turbine reacts only to energy released in the combustors and the fuel flow control process is actually a volumetric flow control process, fuels of different composition with relatively close Wobbe Indices can generally be provided in the same fuel control system. The Wobbe Index is defined most generally as the relative fuel heating value divided by the relative density. A modified Wobbe Index is even more instructive because it takes into account the temperature of the fuel. The Modified Wobbe Index is the ratio of the lower heating value to the square root of the product of the specific gravity and the absolute gas temperature.
Variations in the modified Wobbe Index from the specified value for the fuel supplied can lead to unacceptable levels of combustion dynamics. That is, it has been determined that combustion dynamics may be a function of the modified Wobbe Index. Consequently, operation at high levels of variations in the modified Wobbe Index from a specified value can result in hardware distress, reduced component life of the combustion system and a potential for power generation outage.
Further, the performance of a gas turbine in avoiding combustion dynamics is sensitive to the combination of fuel and fuel nozzle for the combustion. When a gas turbine combustor is tuned to avoid combustion dynamics with a specific nozzle geometry and a gas fuel with a modified Wobbe value requiring high gas fuel temperatures for emissions compliant operation at baseload, operation with cold fuel can lead to combustion dynamics and non-compliant emissions. Consequently, a control system may provide interlocks to prevent shifting to an emissions compliant mode capable of achieving baseload, unless fuel temperature and/or the modified Wobbe index exceeds a designated value or range
Industrial and power generation gas turbines have such control systems with controllers that monitor and control their operation. These controllers govern the combustion system of the gas turbine, controlling various modes of operation from cold startup through baseload. In addition to operating the gas turbine to prevent combustion dynamics during baseload operation, the controller must maintain gas turbine emissions compliant with government regulations and contractual obligations and at the same time promote efficient power output.
Currently certain gas turbines, such as 7FA+e model gas turbines by General Electric Co., which are designed to run on hot gas fuel, are prevented from operating at an emissions compliant combustion mode when the fuel gas temperature is below a specified range and/or the modified Wobbe index is out of range. This limitation prevents high combustion dynamics, which can lead to hardware damage and/or unit flame out. Typically, power plants heat their fuel using a balance of plant processes, which take a significant amount of time to reach operating temperature. The current forced lockout of emissions compliant mode when fuel temperature is below the specified range means the operator cannot reach higher loads and must hold at a low load level, waiting for fuel temperature to increase. Such delays cost the operator time, extending operation under non-emission compliant modes, and loss of power generation revenues.
Accordingly, there is a need to provide a method to allow operation for gas turbines at baseload in an emission compliant mode when gas fuel temperatures are below a normal range for such operation.